1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device of a so-called MONOS (metal oxide nitride oxide silicon) type or the like that has a first insulating film, a charge storage film having a charge storage ability, and a second insulating film between a channel formation region, in which a channel inversion layer is formed, and a control electrode and a method of manufacturing the nonvolatile semiconductor memory device.
2. Description of the Related Art
A so-called MONOS memory transistor is known as a nonvolatile memory device of a trap gate type that stores charges in a charge trap in an insulating film.
Usually, in the case of a MONOS memory transistor of an N-channel type, a so-called ONO (Oxide-Nitride-Oxide) film is formed over the entire region between a P-type semiconductor layer (a P well), which includes a surface region (a channel formation region) in which a channel inversion layer (an N-type inversion layer) is formed, and a gate electrode.
As a method of writing and erasing memory of the MONOS memory transistor, there are the following two methods (a first method and a second method). As an example, bias application conditions for the N-channel type MONOS memory transistor are described below.
In the first method, a high voltage is applied a region between the gate electrode and the channel formation region to inject charges from the entire surface of the channel formation region into, in particular, a nitride film (a form of a charge storage film) in the ONO film using a tunnel current. When injection of electrons is defined as writing and drawing of electrons is defined as erasing, a positive bias is applied to the gate electrode at the time of the writing and a negative bias is applied to the gate electrode at the time of the erasing.
In the second method, the MONOS memory transistor is caused to perform a MOS operation. Injection of electrons is defined as writing and injection of holes electrically offsetting the injected electrons is defined as erasing. In the writing, a positive bias is applied to a drain and a positive bias is applied to a gate with a source as a reference to form a channel inversion layer and electros flowing in the channel inversion layer are made hot and injected into a region near the drain, that is, a local portion of a charge storage film in the ONO film (channel hot electron (CHE) injection). In the erasing, a negative bias is applied to the gate and a positive bias is applied to the drain to inject holes made hot, that is, hot holes caused by, for example, band-to-band tunneling at a drain end into a local portion of the charge storage film (the region near the drain) into which the electrons have already been injected. Consequently, the electrons injected earlier are electrically neutralized and a memory state is erased.
However, in the first method, a high voltage is necessary for the writing and the erasing. In the second method, although a voltage for the writing is low, a large amount of an electric current is necessary for the writing.
It is known that, when the writing (injection of electrons) and the erasing (injection of holes) are repeated using the second method, as the number of times of the erasing (injection of holes) increases, a retention characteristic is deteriorated (a retention time decreases).
As a cause of the deterioration in the holding characteristic, it is considered that, as the number of times of the erasing (injection of holes) increases, residual holes injected into the charge storage film but not used for actual erasing (recombination with electrons) are stored in a place shifted from an injection position of electrons injected, the residual holes spread as time elapses to recombine with the electrons injected, and the holes deteriorate the retention characteristic.
As measures against the deterioration in the retention characteristic, there is known a MONOS memory transistor having a structure in which a composition of a local portion of a film in a lowermost layer (hereinafter referred to as potential barrier film) in the ONO film is made different from that of the other portions (see, for example, JP-A-2005-277032).
In a technique disclosed in JP-A-2005-277032, a composition of a local portion of a potential barrier film and a position and a size (or a plane shape) of the local portion in the potential barrier film are appropriately set such that, for charges that spread when the charges are stored, the distribution of the charges can be narrowed or an amount of injection of the charges can be controlled. As a result, a range of injection of the charges into a charge storage layer is limited to some extent to control the spread of the distribution.